A fluorescent lamp is used as not only a hot electrode fluorescent lamp for general lighting but also a cold cathode fluorescent lamp for a backlight of a liquid crystal display set in a device such as a television or a computer, a scanning light source of a facsimile or an eraser light source of a copying machine, or an external electrode fluorescent. The fluorescent lamp has a light transmitting tube made of a glass with a fluorescent layer on an inner wall surface thereof; and electrodes internally or externally installed to both ends of the light transmitting tube. Mercury and rare gas such as argon are enclosed in the light transmitting tube. Fluorescence may occur in a following way. When a voltage is applied between fluorescent lamp electrodes, electrons emitted in the light transmitting tube ionize rare gas. Then, the ionized rare gas is attracted to the electrodes and in turn secondary electrons are emitted from the electrodes, thereby generating glow discharge. The mercury is excited by the glow discharge to emit ultraviolet ray and then the fluorescent material excited by the ultraviolet ray emits the fluorescence in a range of visible rays.
The electrodes in such a fluorescent lamp are subjected to the sputtering of the mercury or the ionized rare gas so that atoms in the electrodes are beaten out of the electrodes. Accordingly, the electrodes are easily deteriorated, resulting in the shortened life time of the fluorescent lamp. For this reason, material of the electrodes is selected to have excellent sputtering resistance. The material of the electrodes in the fluorescent lamp has employed nickel or nickel alloy because the nickel or nickel alloy has the excellent sputtering resistance and is easy-manufacturing at low cost. However, the nickel atoms beaten out of the electrodes by sputtering tend to react with the mercury to generate amalgam. Further, with the deterioration of the electrodes, the mercury may be consumed to shorten the life time of the fluorescent lamp.
Moreover, the cold cathode fluorescent lamp is frequently used under darkness where it is difficult for external electrons to reach inside the lamp, and, hence, it takes long time for the secondary electrons to be emitted from the electrodes after a start voltage is applied to both electrodes. Such a cold cathode fluorescent lamp, in which hot electrons are not expected to be emitted from the electrodes, may put the light on within 20 to 30 milliseconds under the presence of ambient light after high frequency high voltage in a range of 50 to 60 kHz and 1000 to 1200 V is applied to both electrodes, whereas the cold cathode fluorescent lamp may not put the light on immediately under the darkness, and it takes more than one second to light up. Occasionally it never lights up. In this manner, under the darkness, the cold cathode fluorescent lamp has extremely unstable starting characteristics.
In order to improve such extremely unstable starting characteristics of the cold cathode fluorescent lamp, Patent Documents 1, 2 disclose the discharge lamp including an electron emitter layer made of LaB6 or CeB6 as the electron emitting material formed on the surface of the electrodes.
In such discharge lamps disclosed in the Patent Documents 1, 2, however, the electron emitter layer made of the electron emitting material is formed on the surface of the electrodes, hence, the electron emitting material has been consumed out by the sputtering during the use of the discharge lamps. Accordingly, it has a problem that such discharge lamps do not remain good dark-start characteristics for long periods.